The invention relates generally to assembly-line systems for assembling a plurality of identical structures, and more particularly to the assembly of large mechanical structures such as aircraft wing boxes that are cumbersome to move from one assembly station to another.
Henry Ford is generally regarded as the father of modern mass production. Ford recognized that great strides in efficiency of mass production could be realized by standardizing the design of an automobile so that parts could be interchangeably used from one automobile to another, and by dividing the assembly of an automobile into a number of different tasks. Ford also conceived that workers could perform more efficiently if each repetitively performed the same task or set of tasks on each automobile. Efficiency in the flow of products was improved by stationing workers at fixed workstations arranged along an xe2x80x9cassembly linexe2x80x9d and by moving each automobile along the line from one workstation to the next. Each workstation had the parts and equipment that its workers would need to perform their assigned tasks. Since Ford""s pioneering work, it has become standard across virtually all industries to use moving assembly lines in the mass production of products.
Improvements in manufacturing efficiency and cost have also been achieved by improving the control of parts inventories and flow. One example is the advent of xe2x80x9cjust-in-timexe2x80x9d inventory systems in which parts are replenished in the inventory at the rate they are used, so that a large inventory is not needed. The xe2x80x9ckanbanxe2x80x9d (a Japanese term meaning xe2x80x9creplenishment to orderxe2x80x9d) inventory system is one version of this, in which the usage of each part is communicated from the user to the part supplier as the parts are used, and the supplier delivers parts of the same type and number as are used. In most cases, the delivered parts are received at a loading dock and are then transported from the loading dock to a central parts storage area where all parts used in the manufacture of the particular product are stored and catalogued. Alternatively, the parts can be transported from the loading dock to one or more parts storage areas proximate the point of use.
Moving assembly lines can be difficult to implement when the product is so large or delicate that it cannot be practically moved from one workstation to another. For instance, aircraft wing boxes for large commercial aircraft can be quite large and heavy. Accordingly, wing boxes have traditionally been assembled by keeping the wing boxes in fixed locations. Where the wing boxes are assembled in vertical orientations, custom-built worker platforms are positioned in proximity to a wing box so that workers can access all portions of the wing box. Teams of workers successively perform their assigned tasks on the wing boxes until they are completed. In the process, tools and equipment needed for a given set of tasks are retrieved by the workers from racks stationed on the assembly floor, carried to the wing box being assembled, and used for performing the tasks. Component parts to be installed in a wing box are generally stored in a number of parts storage racks located in the assembly building. Workers retrieve the parts from the racks and the parts are carried or transported to the wing box. Parts are replenished in the racks by transporting them from a part-receiving area (i.e., a loading dock) to the racks and placing them in their proper locations so that they can be subsequently retrieved. In most cases, the parts racks are not immediately adjacent to the wing box being worked on, and hence, workers must travel to the racks to retrieve the needed parts. Likewise, tools are typically not stored immediately adjacent to the wing box location, and thus the workers must travel to the tool storage area to retrieve needed tools.
The assembly method described above has certain drawbacks. One significant disadvantage is the time that is wasted by workers traveling from the assembly location to a tool or parts storage rack, retrieving the needed tools and/or parts, and traveling back to the assembly location. This inefficiency could be substantially reduced by storing the tools and parts at the point of use. In many cases, however, it may be impossible or undesirable to use space on the assembly floor for such purpose; for example, the cost per square foot for an assembly building is often much higher than that for a parts warehouse, such that it may be economically disadvantageous to store parts at the point of use.
A further drawback in the above-described assembly system is that elaborate and expensive positioning and fixturing devices have generally been necessary for precisely positioning and holding component parts in their proper locations so that they can be drilled and fastened to the wing box. For example, in many determinant assembly processes for large structures such as wing boxes, large 5-axis gantry machines are used, sometimes in conjunction with laser tracking devices, for positioning component parts and fastening the parts to the structure. These devices are relatively expensive.
The above needs are met and other advantages are achieved by the present invention, which provides a system and method for the assembly of large mechanical structures that cannot practically be moved from one workstation to another. In accordance with the principles of the present invention, the mechanical structures to be assembled are fixed in stationary assembly jigs spaced apart on an assembly floor. A plurality of different, specialized work operations are performed on each structure by successively moving a plurality of mobile workstations to the structure. Each workstation is designed to facilitate a particular set of work operations. In one embodiment of the invention for use in assembling aircraft wing boxes, the workstations can include a drilling and fastening workstation, a systems installation workstation, a functional test workstation, and a self-contained cleaning workstation. Each workstation comprises a mobile module that can travel along the assembly floor from one assembly jig to the next. One or more of the workstations include a worker platform facilitating worker access to the structure being assembled.
Advantageously, at least one of the workstations includes a parts storage module for storing component parts to be installed in the mechanical structures. The required tools for the assembly tasks can also be stored in the parts storage module. Thus, the parts and tools needed for performing a given set of tasks are located immediately adjacent the point of use, thereby substantially reducing the time required for retrieving parts and tools. Preferably, the parts storage module(s) can be moved to a part-receiving area (e.g., a loading dock) for directly receiving parts when they are delivered by the supplier, thereby eliminating central inventory storage. The parts storage module(s) can interface with the worker platform(s) so that workers on a worker platform can easily retrieve parts or tools from the parts storage module without leaving the platform.
In one embodiment of the invention, one or more of the workstations include docking members and the assembly jigs include cooperating docking members that engage the docking members of the one or more workstations for aligning the workstations with the assembly jigs. This enables the workstations to index to the assembly jigs so that part positioning and fixturing devices mounted on the workstations can accurately position component parts relative to the mechanical structure.
Advantageously, at least one workstation includes one or more indexing devices for positioning components to be installed in the mechanical structure so that the components can be affixed to the mechanical structure. The indexing devices can comprise indexing arms that are mounted on a mobile worker platform. The platform indexes to the assembly jig, preferably by way of the docking members, so that the indexing arms can repeatably and accurately position components relative to the mechanical structure. As an alternative to indexing arms, other indexing devices such as laser trackers or photographic measurement systems can be mounted on the worker platform for positioning parts.
In one embodiment, at least one workstation comprises a fixturing apparatus operable to hold a subassembly to be secured to the mechanical structure, and operable to transport the subassembly to one of the assembly jigs and to position the subassembly relative to the mechanical structure such that the subassembly can be attached to the mechanical structure.
In many cases, one or more of the workstations will require a supply of electrical power for operating equipment on the workstation. Advantageously, each of the assembly jigs includes an electrical coupling member connected to a source of electrical power, and each workstation requiring electrical power includes a cooperating electrical coupling member operable to mate with the electrical coupling member of each assembly jig when the workstation docks with the assembly jig, whereby electrical power is supplied from the assembly jig to the workstation. The assembly jigs can also have fluid coupling members and/or data coupling members that mate with corresponding fluid and data coupling members in one or more workstations so that fluid and/or data transfer between the workstation and the assembly jig can occur. For example, pneumatic tools such as drills can be used in a workstation, and the air supply for the tools can be delivered through an air supply coupling member on the assembly jig that mates with an air supply coupling member on the workstation.
In one embodiment of the invention, one of the workstations comprises a cleaning station operable to perform cleaning operations on the mechanical structure. The cleaning station preferably comprises two traveling modules operable to engage each assembly jig from opposite sides thereof. Each module includes half of a containment shell, and the mechanical structure is enclosed in the containment shell when the modules engage the opposite sides of the assembly jig. Automatic spray cleaning of the mechanical structure can be performed within the containment shell to sluice debris and deposits from the structure.
One workstation can comprise a testing module operable to perform functional testing operations on the mechanical structure. Preferably, the testing module includes equipment for functional testing of all electrical and fluid systems installed in the mechanical structure. The time-wasting practice of retrieving test equipment from various locations around the assembly area is thereby eliminated.
When the invention is adapted to the assembly of aircraft wing boxes, there preferably are a plurality of vertical assembly jigs stationed on the assembly floor for fixturing the wing boxes in vertical orientations. The mobile worker platforms interface with the assembly jigs so that workers have access to the opposite major surfaces of the wing boxes. The worker platforms are of a common basic design, which is then customized for each workstation by installing data stations, work benches, utility ports, and other features, that are adapted to the particular assembly operations to be performed. The parts storage modules can interface with the worker platforms, and are also of a common basic design that is then customized with appropriate shelving and cabinetry for containing the particular parts and tools needed for each workstation.
Wing substructure fit-up is accomplished by a tooless indexing workstation comprising a worker platform on which are mounted one or more three-dimensional indexing devices for positioning the substructure. The tooless indexing workstation indexes to the assembly jigs by way of a platform interlock system including docking members on the platform and the assembly jigs that interlock to align the platform with the jigs.